西藏冈底斯矿带南缘矽卡岩型铜矿床含矿岩体锆石U-Pb年龄及意义

西藏冈底斯矿带发育大量斑岩铜钼矿床及铜铅锌多金属矿床,形成斑岩铜矿带及多金属矿带。过去的工作表明,冈底斯带南部矿床同位素年龄多小于30Ma,形成于碰撞期后伸展环境。本文测定了冈底斯矿带南缘克鲁-冲木达矽卡岩型铜(金、钼)矿集区桑布加拉矽卡岩型铜(金)矿化岩体锆石LA-ICP-MSU-Pb年龄及锆石Ce4+/Ce3+比值。矿化岩体锆石U-Pb年龄:92.1±0.6Ma,MSWD=1.0,锆石Ce4+/Ce3+比值在90～562之间,平均值为287。锆石Ce4+/Ce3+比值和玉龙矿带含矿岩体锆石的比值基本一致,显示矽卡岩矿化岩体岩浆氧逸度较高。印度板块与欧亚板块碰撞时间在65～45Ma之间,桑布加拉矽卡岩型铜矿化岩体锆石U-Pb年龄表明冈底斯带不但发育碰撞期后大规模成矿作用,也发育与洋壳俯冲构造岩浆事件有关的成矿作用。这为冈底斯矿带洋壳俯冲有关矿床的寻找提供了依据。     

The adakite connection of the Tuwu–Yandong copper porphyry belt, eastern Tianshan, NW China: trace element and Sr-Nd-Pb isotope geochemistry

The Tuwu–Yandong porphyry copper belt lies in the eastern Tianshan mountains, eastern section of the Central Asian orogenic belt. The copper mineralization is mainly hosted in plagiogranite porphyries intruded into early Carboniferous volcanic rocks of the Paleozoic Dananhu island arc between the Tarim and Siberian plates. The plagiogranite porphyries have contents of 65–73 wt% SiO₂, 14–17 wt% Al₂O₃, 0.9–2.2 wt% MgO, 3–16 ppm Y, 0.4–1.40 ppm Yb, 347–920 ppm Sr, and positive Eu anomalies. The rocks also exhibit positive ɛ _Nd(t) values (+5.0 to +9.4) and low initial ⁸⁷Sr/⁸⁶Sr values (0.70316–0.70378). Such features are similar to those of adakites derived from partial melting of a subduction-related oceanic slab. The mineralization age is early Carboniferous (350–320 Ma), which is close to that of the porphyries. The close relationship between the Cu mineralization and the porphyry is also indicated by their similar Sr-Nd-Pb isotopic compositions. We suggest that the copper porphyry (magma) system in the Dananhu island arc was formed by direct melting of an obliquely subducting early Carboniferous oceanic slab.     

Characteristics of Rare Earth Elements,Zr, and Hf in Ore‐Bearing Porphyries from the Western Awulale Metallogenic Belt,Northwestern China and their Application in Determining Metal Fertility of Granitic Magma

The western Awulale metallogenic belt in northwestern China hosts a number of small‐ to medium‐sized porphyry Cu deposits that are associated with albite porphyry. The common presence of plagioclase (albite) as phenocrysts and the absence of hydrous minerals (amphibole and biotite) indicate that the water content of albite porphyry is low. Trace‐element compositions of whole rocks and zircon grains from these ore‐bearing porphyries were measured. Zircon grains from albite porphyries exhibit Ce⁴⁺/Ce³⁺ ratios ranging from 7.75 to 95.1, which indicate that these porphyries have a low oxygen fugacity. Trace element compositions of ore‐bearing porphyries exhibit (La/Yb)_N ratios ranging from 1.09 to 11.1 and Eu/Eu^* ratios ranging from 0.10 to 0.66. These ore‐bearing porphyries have Zr values ranging from 171 to 707 ppm and Hf values ranging from 8.30 to 18.9 ppm. Combining these porphyries with other ore‐bearing porphyries that formed in the Central Asian Orogenic Belt (CAOB) and the Pacific Rim metallogenic belt, we found that the (La/Yb)_N and Eu/Eu^* ratios of ore‐bearing porphyries in western Awulale are low, while the Zr and Hf values are high. Specifically, REEs can be used to evaluate the degree of differentiation and degree of partial melting, and Zr and Hf can be used to evaluate the redox conditions and water content of magmatic rocks. Our findings indicate that ore‐bearing porphyries in western Awulale have a lower oxygen fugacity, degree of differentiation, and water content than do others in the CAOB and the Pacific Rim metallogenic belt. Compared to those of ore‐bearing porphyries with lower zircon Ce⁴⁺/Ce³⁺ ratios, the (La/Yb)_N ratios of ore‐bearing porphyries in our study are low, and the Zr and Hf values are high. This finding indicates that, under reducing conditions, the degree of evolution and water content may have an important influence on the metal abundance in magmas. There is also a clear relationship between (La/Yb)_N, Eu/Eu^*, Zr, Hf, and the size of the deposits. Large‐ (>4 Mt Cu) and intermediate (1.5–4 Mt Cu)‐sized porphyry Cu deposits are associated with granitic intrusions that have higher (La/Yb)_N and Eu/Eu^* ratios and lower Zr and Hf values. This finding indicates that, in addition to oxygen fugacity, the degree of evolution and water content are controlling parameters for metal abundance in magmas, especially in low oxygen fugacity porphyry Cu deposits. Such a conclusion may be useful in the exploration for other concealed porphyry Cu deposits.     

西藏甲玛超大型铜矿区斑岩脉成岩时代及其与成矿的关系

甲玛铜多金属矿是冈底斯成矿带上资源储量达到超大型规模的又一个重要矿床,2010年7月已正式投产。产于矽卡岩、斑岩和角岩中的辉钼矿Re-Os定年已表明甲玛矿床的铜钼成矿时代集中于17~14Ma,而成岩时代的研究相对较少,尤其是矿区及外围大量出露的近南北向展布的斑岩脉。本文选取矿区铅山上52号平硐内的2件弱矿化斑岩脉样品,花岗斑岩(JM52-0)和花岗闪长斑岩(JM52-46.7),首次开展斑岩脉的锆石SHRIMP U-Pb定年,获得的²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U协和年龄分别为14.2±0.2Ma和14.1±0.3Ma,代表了甲玛矿区地表出露的近南北向展布的斑岩脉侵位时岩浆锆石的结晶年龄。斑岩脉的成岩时代与区域上与近南北向正断层系统及裂谷裂陷带有关的冈底斯含矿斑岩侵位时代(18~12Ma)一致。甲玛的成岩成矿时代显示了成岩作用与成矿作用基本同期,且与冈底斯成矿带东段主要斑岩型-矽卡岩型铜多金属矿床的成岩成矿时代基本一致,成矿高峰集中在17~14Ma之间,指示了冈底斯在中新世的岩浆构造活动事件,而且表明了甲玛铜钼矿化与岩浆热液的成因联系。     

Geology,geochronology and geochemistry of granitic intrusions and the related ores at the Hongshan Cu-polymetallic deposit: Insights into the Late Cretaceous post-collisional porphyry-related mineralization systems in the southern Yidun arc,SW China

The Hongshan Cu-polymetallic deposit is located in the southern Yidun arc in southwestern China, where both subduction-related (Late Triassic) and post-collisional (Late Cretaceous) porphyry–skarn–epithermal mineralization systems have been previously recognized. In this study, two distinct magmatic events, represented by diorite porphyry and quartz monzonite porphyry, have been revealed in the Hongshan deposit, with zircon SHRIMP U–Pb ages of 214 ± 2 Ma and 73.4 ± 0.7 Ma, respectively. The 73 Ma age is comparable to the Re–Os ages of 77 to 80 Ma of ore minerals from the Hongshan deposit, indicating that the mineralization is related to the Late Cretaceous quartz monzonite porphyries rather than Late Triassic diorite porphyries. The Late Triassic diorite porphyries belong to the high-K calc-alkaline series and show arc magmatic geochemical characteristics such as enrichment in Rb, Ba, Th and U and depletion in HFSEs, indicating that they were formed during the westward subduction of the Garzê–Litang Ocean. In contrast, the Late Cretaceous quartz monzonite porphyries show shoshonitic I-type geochemical characteristics, with high SiO₂, K₂O, LILE, low HREE, Y and Yb contents, and high LREE/HREE and La/Yb ratios. These geochemical characteristics, together with the Sr–Nd–Pb isotopic compositions (average (⁸⁷Sr/⁸⁶Sr)_i = 0.7085; ε_Nd(t) = − 6.0; ²⁰⁶Pb/²⁰⁴Pb = 19.064, ²⁰⁷Pb/²⁰⁴Pb = 15.738, ²⁰⁸Pb/²⁰⁴Pb = 39.733) suggest that the quartz monzonite porphyries originated from the partial melting of the ancient lower crust in response to underplating of mafic magma from subduction metasomatized mantle lithosphere, possibly triggered by regional extension in the post-collisional tectonic stage. The S isotopic compositions (δ³⁴S_V-CDT = 3.81‰ to 5.80‰) and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.014 to 18.809, ²⁰⁷Pb/²⁰⁴Pb = 15.550 to 15.785, and ²⁰⁸Pb/²⁰⁴Pb = 38.057 to 39.468) of ore sulfides indicate that the sulfur and metals were derived from mixed mantle and crustal sources. It is proposed that although the Late Triassic magmatic event is not directly related to mineralization, it contributed to the Late Cretaceous mineralization system through the storage of large amounts of sulfur and metals as well as water in the cumulate zone in the mantle lithosphere through subduction metasomatism. Re-melting of the mantle lithosphere including the hydrous cumulate zone and ancient lower crust during the post-collisional stage produced fertile magmas, which ascended to shallow depths to form quartz monzonite porphyries. Hydrothermal fluids released from the intrusions resulted in porphyry-type Mo–Cu ores in and near the intrusions, skarn-type Cu–Mo ores in the country rocks above the intrusions, and hydrothermal Pb–Zn ores in the periphery.     

Melt components derived from a subducted slab in late orogenic ore-bearing porphyries in the Gangdese copper belt, southern Tibetan plateau

A porphyry-type Cu (Mo, Au) polymetallic ore belt extends in an E–W direction for >400 km along the Gangdese magma arc in the southern Tibetan plateau. Extensive field investigations and systematic geochemical study, combined with S and Pb isotopic tracing, indicate that Cu polymetallic mineralization of the copper belt is genetically related to late orogenic granitic porphyries formed in a late Himalayan crustal extension stage (18–14 Ma). Geochemistry of the porphyries shows the essential characteristics of adakite and suggests a dominant role of subduction-related components in their genesis. The genesis of these ore-bearing porphyries is mainly related to melt components derived from subducted slabs and has little relationship with fluids released from them. Pb isotopes show a clear linear array in the plumbotectonics model and are manifested by a progressive variation from orogenic Pb in the east segment of the copper belt to mantle Pb in the west segment, forming a mixing line of Indian Oceanic MORB and Indian Oceanic sediments. This suggests that the porphyry magma dominantly originated from partial melting of subducted oceanic crust and was mixed with a minor melts of sediments and mantle wedge components.     

Re–Os molybdenite ages and zircon Hf isotopes of the Gangjiang porphyry Cu–Mo deposit in the Tibetan Orogen

The Miocene porphyry Cu–(Mo) deposits in the Gangdese orogenic belt in southern Tibet were formed in a post-subduction collisional setting. They are closely related to the Miocene adakite-like porphyries which were probably derived from a thickened basaltic lower crust. Furthermore, mantle components have been considered to have played a crucial role in formation of these porphyry deposits (Hou et al. Ore Geol Rev 36: 25–51, 2009; Miner Deposita doi:10.1007/s00126-012-0415-6, 2012). In this study, we present zircon Hf isotopes and molybdenite Re–Os ages on the newly discovered Gangjiang porphyry Cu–Mo deposit in southern Tibet to constrain the magma source of the intrusions and the timing of mineralization. The Gangjiang porphyry Cu–Mo deposit is located in the Nimu ore field in the central Gangdese porphyry deposits belt, southern Tibet. The copper and molybdenum mineralization occur mainly as disseminations and veins in the overlapped part of the potassic and phyllic alteration zones, and are predominantly hosted in the quartz monzonite stock and in contact with the rhyodacite porphyry stock. SIMS zircon U–Pb dating of the pre-mineral quartz monzonite stock and late intra-mineral rhyodacite porphyry yielded ages of 14.73?±?0.13 Ma (2σ) and 12.01?±?0.29 Ma (2σ), respectively. These results indicate that the magmatism could have lasted as long as about 2.7 Ma for the Gangjiang deposit. The newly obtained Re–Os model ages vary from 12.51?±?0.19 Ma (2σ) to 12.85?±?0.18 Ma (2σ) for four molybdenite samples. These Re–Os ages are roughly coincident with the rhyodacite porphyry U–Pb zircon age, and indicate a relatively short-lived episode of ore deposition (ca. 0.3 Ma). In situ Hf isotopic analyses on zircons by using LA-MC-ICP-MS indicate that the ε _Hf(t) values of zircons from a quartz monzonite sample vary from +2.25 to +4.57 with an average of +3.33, while zircons from a rhyodacite porphyry sample vary from +5.53 to +7.81 with an average of +6.64. The Hf data indicate that mantle components could be partly involved in the deposit formation, and that mantle contributions might have increased over time from ca. 14.7 to 12.0 Ma. Combined with previous works, it is proposed that the Gangjiang deposit could have resulted from the convective thinning of the lithospheric root, and the input of upper mantle components into the magma could have played a key role in the formation of the porphyry deposits in the Miocene Gangdese porphyry copper belt in the Tibetan Orogen.     

Genesis of Luobuzhen Pb–Zn veins: Implications for porphyry Cu systems and exploration targeting at Luobuzhen-Dongshibu in western Gangdese belt,southern Tibet

Porphyry systems are known to form in magmatic arc environment and commonly include porphyry Cu, epithermal Pb–Zn–Au–Ag, skarn polymetallic mineralization, etc. The systems are rarely reported in collisional zones, such as the Gangdese belt in southern Tibet where many postcollisional porphyry copper deposits occurred. In addition, other types of mineral systems are rarely present except porphyry copper mineralization in the Gangdese belt. In this study, we present Pb–Zn-bearing quartz veins at Luobuzhen in the western Gangdese belt. The Luobuzhen Pb–Zn veins cross-cut dacite of the Linzizong Group with zircon U–Pb age of 50.1 ± 0.2 Ma and monzogranite with zircon U–Pb age of 17.1 ± 0.1 Ma. Ore minerals include sphalerite, galena, chalcopyrite, and pyrite; gangue minerals are quartz with minor chlorite and sericite. Primary fluid inclusions of quartz are liquid-rich, aqueous, and two-phase inclusions. The homogenization temperatures of these primary inclusions are moderate to high (267–400 °C), and salinities range from 8.9 to 18.4 wt.% NaCl equiv. Quartz has δ¹⁸O_SMOW values of 6.2–9.3‰, while sulfides have δ³⁴S_V-CDT values of −5.1‰ to 0.1‰, ²⁰⁶Pb/²⁰⁴Pb of 18.722–18.849, ²⁰⁷Pb/²⁰⁴Pb of 15.640–15.785, and ²⁰⁸Pb/²⁰⁴Pb of 39.068–39.560. These data suggest that magmatic fluids with contribution from meteoric water, magmatic sulfur, and lead derived from upper crust and metasomatized mantle by Indian continental materials would be critical for the Luobuzhen base metal mineralization.The Dongshibu area, located at ∼2 km east of the Luobuzhen, is characterized by high concentrations of Cu (up to 1450 ppm) and Mo (up to 130 ppm) of stream sediments, which is quite different from high concentrations in Pb, Zn, Ag, and Au shown in the Luobuzhen area. In addition, porphyry copper mineralization-related alteration and veins/veinlets occur in the Miocene monzogranite at Dongshibu. The monzogranite is characterized by high Sr/Y ratios, which are also shown on ore-forming intrusions in the Gangdese postcollisional porphyry copper deposits, and shows similar zircon Hf isotopes to the ore-related high Sr/Y intrusions from the Zhunuo porphyry copper deposit which is located ∼20 km northeast of the Luobuzhen-Dongshibu. A comprehensive analysis allows us to infer that the base metal veins at Luobuzhen are components of a porphyry Cu system with porphyry Cu mineralization likely present at Dongshibu and epithermal Au–Ag veins possibly occurring at Luobuzhen, which are indicative of the existence of porphyry copper systems in collisional zones. The potential porphyry Cu mineralization and epithermal Au–Ag veins should be targeted in future exploration at Luobuzhen-Dongshibu.     

High oxidation magmatic evolution in the Naruo porphyry Cu deposit,Tibet, China

The Naruo porphyry Cu deposit is the third largest deposit discovered in the Duolong metallogenic district. Previous research has focused mainly on the geochemistry of the ore-bearing granodiorite porphyry; the metallogenesis remains poorly understood. In the present work, on the basis of outcrops and drilling core geological mapping, phases of early mineralization diorite, two inter-mineralization granodiorite porphyries, and late-mineralization granodiorite porphyry have been distinguished. Furthermore, the alteration zones were outlined, and the vein sequence was identified. The diorite and three porphyry phases were subjected to Laser Ablation Inductively Coupled Plasma Mass Spectrometry (La–ICP–MS) zircon U–Pb dating and in situ Hf isotope analyses as well as bulk major element, trace element, and Sr–Nd isotopic analyses. Molybdenite Re–Os dating was also conducted.The zircon U–Pb dating results show that the diorite and porphyry intrusions were emplaced at about 120 Ma, and the molybdenite Re–Os isochron age is 118.8 ± 1.9 Ma; this indicates that the Naruo porphyry Cu deposit was formed during a continuous magmatic–hydrothermal process. All of the diorite and granodiorite porphyry samples showed arc magmatic characteristics. Moreover, the moderate (⁸⁷Sr/⁸⁶Sr)_i ratios and low ε_Nd(t) and ε_Hf(t) values of the diorite and porphyry intrusions suggest the source region of the juvenile lower crust. The lower (⁸⁷Sr/⁸⁶Sr)_i and (¹⁴³Nd/¹⁴⁴Nd)_i ratios and higher ε_Nd(t) values and incompatible element concentrations than those in the granodiorite porphyry samples indicate a two-stage magmatic generation process for the intrusions. The early mineralization diorite has a high Cu concentration, implying that the source is enriched in Cu. However, the slightly lower Cu content of the late-mineralization granodiorite porphyry samples might imply Cu release from magmas and deposition within the metallogenic stage. The multiple stages of intrusions and subsequent volcanism within the Duolong metallogenic district, together with high Sr/Y features, indicate persistent magmatism during the metallogenic epoch, which is necessary for maintaining the activity of magmatic–hydrothermal and mineralization processes. Thus, the high Cu content in the source region, mantle-derived melt upwelling, and multiple stages of persistent magmatism were favorable for the formation of the Naruo porphyry Cu deposit.The high Fe₂O₃/FeO ratios of the diorite and granodiorite porphyry intrusions show very high oxidation features, which is coincident with estimated magmatic oxidation state calculated by the zircon trace element compositions. The high oxidation facilitates sulfur and chalcophile metals to be scavenged into the magmatic–hydrothermal systems, which is crucial for the metallogenesis of the Naruo porphyry Cu deposit.     

Origin of the Newly Discovered Zhunuo Porphyry Cu-Mo-Au Deposit in the Western Part of the Gangdese Porphyry Copper Belt in the Southern Tibetan Plateau, SW China

The newly discovered Zhunuo porphyry Cu-Mo-Au deposit is located in the western part of the Gangdese porphyry copper belt in southern Tibet, SW China. The granitoid plutons in the Zhunuo region are composed of quartz diorite porphyry, diorite porphyry, granodiorite porphyry, biotite monzogranite and quartz porphyry. The quartz diorite porphyry yielded zircon U-Pb ages of 51.9±0.7 Ma(Eocene) using LA-ICP-MS, whereas the diorite porphyry, granodiorite porphyry, biotite monzogranite and quartz porphyry yielded ages ranging from 16.2±0.2 to 14.0±0.2 Ma(Miocene). CuMo-Au mineralization is mainly hosted in the Miocene granodiorite porphyry. Samples from all granitoid plutons have geochemical compositions consistent with high-K calc-alkaline series magmatism. The samples display highly fractionated light rare-earth element(REE) distributions and heavy REE distributions with weakly negative Eu anomalies on chondrite-normalized REE patterns. The trace element distributions exhibit positive anomalies for large-ion lithophile elements(Rb, K, U, Th and Pb) and negative anomalies for high-field-strength elements(Nb and Ti) relative to primitive mantlenormalized values. The Eocene quartz diorite porphyry yielded εNd(t) values ranging from-3.6 to-5.2,(~(87)Sr/~(86)Sr)i values in the range 0.7046–0.7063 and initial radiogenic Pb isotopic compositions with ranges of 18.599–18.657 ~(206)Pb/~(204)Pb, 15.642–15.673 ~(207)Pb/~(204)Pb and 38.956–39.199 ~(208)Pb/~(204)Pb. In contrast, the Miocene granitoid plutons yielded ε_(Nd)(t) values ranging from-6.1 to-7.3 and(87Sr/86Sr)i values in the range 0.7071–0.7078 with similar Pb isotopic compositions to the Eocene quart diorite. The Sr-Nd-Pb isotopic compositions of the rocks are consistent with formation from magma containing a component of remelted ancient crust. Zircon grains from the Eocene quartz diorite have ε_(Hf)(t) values ranging from-5.2 to +0.9 and two-stage Hf model ages ranging from 1.07 to 1.46 Ga, while zircon grains from the Miocene granitoid plutons have ε_(Hf)(t) values from-9.9 to +4.2 and two-stage Hf model ages ranging from 1.05–1.73 Ga, indicating that the ancient crustal component likely derives from Paleo- to Mesoproterozoic basement. This source is distinct from that of most porphyry Cu-Mo-Au deposits in the eastern part of the Gangdese porphyry copper belt, which likely originated from juvenile crust. We therefore consider melting of ancient crustal basement to have contributed significantly to the formation Miocene porphyry Cu-Mo-Au deposits in the western part of the Gangdese porphyry copper belt.     

Late Triassic crustal growth in southern Tibet: Evidence from the Gangdese magmatic belt

The Gangdese magmatic belt, located in the southern margin of the Lhasa terrane and carrying significant copper and polymetallic mineralization, preserves important information relating to the tectonics associated with Indian–Eurasian collision and the crustal growth of southern Tibet. Here we investigate the Quxu batholith in the central domain of the Gangdese magmatic belt and report the occurrence of hornblende gabbros for the first time. We present petrologic, zircon U–Pb–Hf isotopic and bulk-rock chemistry data on these rocks. The hornblende gabbros display sub-alkaline features, and correspond to tholeiite composition. They also show medium K calc-alkaline to low K affinity. The rocks show enrichment in LILEs and LREEs, but are depleted in HFSEs, indicating a subduction-related active continental margin setting for the magma genesis. Our computations show that the gabbroic pluton was emplaced in the middle-lower crustal depth of ca. 18 km. Zircons from the hornblende gabbros yield crystallization age of ca. 210 Ma, revealing a late Triassic magmatic event. Combined with available data from the Gangdese magmatic belt, our study suggests that the northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane might have been initiated not later than the Norian period of Triassic. Zircons from the hornblende gabbro show positive ε_Hf(t) values of 9.56 to 14.75 (mean value 12.44), corresponding to single stage model ages (T_DM1) in the range of 256 Ma to 459 Ma, attesting to crustal growth in the southern Lhasa terrane associated with the subduction of the Neo-Tethys oceanic crust.     

Geochronology and magmatic oxygen fugacity of the Tongcun molybdenum deposit, northwest Zhejiang, SE China

The Tongcun Mo porphyry deposit in northwest Zhejiang is hosted in three porphyry units: Huangbaikeng, Songjiazhuang, and Tongcun, from southwest to northeast. U–Pb zircon ages of 162?±?3.0 Ma for the Huangbaikeng porphyry, 159.9?±?3.0 Ma for the Songjiazhuang porphyry, and 167.6–155.6 Ma for the Tongcun porphyry indicate that these intrusions formed during the Jurassic and are most likely associated with the northwestward subduction of the Izanagi Plate. Trace element compositions of zircons from the Tongcun deposit constrain the oxygen fugacity (fO₂) of the magma using zircon Ce anomalies and Ti-in-zircon temperatures. The average magmatic fO₂ for the porphyries in the Tongcun deposit is fayalite–magnetite–quartz (FMQ)?+?2.7, which is similar to the Shapinggou (FMQ?+?3.2) and Dabaoshan (FMQ?+?3.5) Mo porphyry deposits, but much higher than that of the reduced Cretaceous ore-barren Shangjieshou porphyry (FMQ-1.1) around 8 km away from the Tongcun deposit. The distinct difference in magmatic oxygen fugacity between the Jurassic and Cretaceous porphyries may help to explain the absence of Mo porphyry mineralization in northwest Zhejiang during the Cretaceous.     

西藏恰功铁矿二长花岗斑岩锆石的U-Pb年代学与地球化学特征及意义

恰功矽卡岩型Fe(Cu)-PbZn(Ag)矿床的形成与二长花岗斑岩关系密切。该矿床与查藏措、斯弄多、加多捕勒、青都、那扎等矿床已初步在冈底斯中段勾勒出1条矽卡岩成矿带,但目前对这些矿床的研究还较少。本文对发育于西藏恰功矿区南部的2种斑岩的锆石采用CL和LA-ICP-MS进行了成因矿物学和微区微量元素及U-Pb年代学研究,获得石英斑岩的侵位时代为66.83±0.72Ma (MSWD=2.4,n=9),与成矿关系最为密切的二长花岗斑岩侵位时代为67.42±0.80Ma (MSWD=3.8,n=15)。2种斑岩锆石的U/Yb-Hf及U/Yb-Y在微量元素图解中均显示结晶于陆壳环境。二长花岗斑岩中锆石的矿物结构和Zr/Hf、Th/U、Nb/Ta、Nd/Yb值等微量元素特征和根据Ti含量估算的T_Tiz显示,其是源于上地幔基性岩浆脉动上涌过程中同化、混合地壳物质后形成的岩浆熔体,伴随围岩压力降低在上侵过程中结晶、分异,最终于近地表冷却形成的。这些结果暗示恰功矿床的形成与印亚陆陆初始碰撞时回卷的新特提斯洋壳撕裂诱发的壳幔混源岩浆活动有关,代表了一种尚未充分认识的陆陆初始碰撞阶段壳幔混源岩浆活动有关的成矿作用。在冈底斯中段中北部针对该时代矿床的勘查工作有一定的找矿潜力。     

Petrogenesis and metallogenic setting of porphyries of the Duobuza porphyry Cu–Au deposit,central Tibet,China

Given that the Duobuza deposit was the first porphyry Cu–Au deposit discovered in central Tibet, the mineralization and mineralized porphyry in this area have been the focus of intensive research, yet the overall porphyry sequence associated with the deposit remains poorly understood. New geological mapping, logging, and sampling of an early granodiorite porphyry, an inter-mineralization porphyry, and a late-mineralization diorite porphyry were complemented by LA–ICP–MS zircon dating, whole-rock geochemical and Sr–Nd isotopic analyses, and in situ Hf isotopic analyses for both inter- and late-mineralization porphyry intrusions. All of the porphyry intrusions are high-K and calc-alkaline, and were emplaced at ca. 120 Ma. The geochemistry of these intrusions is indicative of arc magmatism, as all three porphyry phases are enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy rare earth elements and high field strength elements. These similar characteristics of the intrusions, when combined with the relatively high (⁸⁷Sr/⁸⁶Sr)_i, negative ε_Nd(t), and positive ε_Hf(t) values, suggest that the magmas that formed the porphyries were derived from a common source region and shared a single magma chamber. The magmas were generated by the mixing of upwelling metasomatized mantle-wedge-derived mafic magmas and magmas generated by partial melting of amphibolite within the lower crust.The inter-mineralization porphyry has the lowest ε_Nd(t) and highest (⁸⁷Sr/⁸⁶Sr)_i values, suggesting that a large amount of lower-crust-derived material was incorporated into the melt and that metals such as Cu and Au from the enriched lower crust were scavenged by the parental magma. The relative mafic late-mineralization diorite porphyry phase was formed by the residual magma in the magma chamber mixing with upwelling mafic melt derived from metasomatized mantle. The magmatic–hydrothermal evolution of the magma in the chamber released ore-forming fluid that was transported mainly by the inter-mineralization porphyry phase during the mineralization stage, which ultimately formed the Duobuza porphyry Cu–Au deposit.These porphyritic intrusions of the Duobuza deposit have high Mg^# and low (La/Yb)_N values, and show some high LILE/HFSE ratios, indicating the magma source was enriched by interaction with slab-derived fluids. Combined with age constraints on the regional tectonic evolution, these dating and geochemical results suggest that the Duobuza porphyry Cu–Au deposit formed in a subduction setting during the final stages of the northward subduction of the Neo-Tethyan Ocean.     

Ages and Sources of Ore‐Related Porphyries at Yongping Cu–Mo Deposit in Jiangxi Province,Southeast China

Whole‐rock geochemistry, zircon U–Pb and molybdenite Re–Os geochronology, and Sr–Nd–Hf isotopes analyses were performed on ore‐related dacite porphyry and quartz porphyry at the Yongping Cu–Mo deposit in Southeast China. The geochemical results show that these porphyry stocks have similar REE patterns, and primitive mantle‐normalized spectra show LILE‐enrichment (Ba, Rb, K) and HFSE (Th, Nb, Ta, Ti) depletion. The zircon SHRIMP U–Pb geochronologic results show that the ore‐related porphyries were emplaced at 162–156 Ma. Hydrothermal muscovite of the quartz porphyry yields a plateau age of 162.1 ± 1.4 Ma (2σ). Two hydrothermal biotite samples of the dacite porphyry show plateau ages of 164 ± 1.3 and 163.8 ± 1.3 Ma. Two molybdenite samples from quartz+molybdenite veins contained in the quartz porphyry yield Re–Os ages of 156.7 ± 2.8 Ma and 155.7 ± 3.6 Ma. The ages of molybdenite coeval to zircon and biotite and muscovite ages of the porphyries within the errors suggest that the Mo mineralization was genetically related to the magmatic emplacement. The whole rocks Nd–Sr isotopic data obtained from both the dacite and quartz porphyries suggest partial melting of the Meso‐Proterozoic crust in contribution to the magma process. The zircon Hf isotopic data also indicate the crustal component is the dominated during the magma generation.     

中甸岛弧成矿斑岩的锆石年代学及其意义

义敦岛弧南段的中甸岛弧印支期浅成-超浅成斑岩体与成矿密切相关,本文对该地区代表性的成矿斑岩如西侧的雪鸡坪、东侧的欠虽等岩体开展了高精度的LA-ICPMS锆石U-Pb定年,获得雪鸡坪成矿斑岩的年龄为213.4±1.5Ma,欠虽石英闪长玢岩年龄为217.1±1.5Ma。本研究和近几年发表的高精度锆石U-Pb年代学成果发现前人划分的中甸岛弧地区东、西成矿斑岩带的斑岩体形成时代没有明显的差异,这些斑岩体呈面状分布,其形成时代集中在223~211Ma,应该属于同一构造作用的产物。同时,这些成矿斑岩的锆石REE配分模式图显示一致的成分趋势,具有强烈的Ce正异常和相对较低的Eu负异常,指示出岩浆过程具有高的氧逸度。结合它们埃达克质的地球化学研究成果,我们认为中甸岛弧成矿斑岩以及相关的斑岩矿床是在东侧的甘孜-理塘洋晚三叠世俯冲过程中形成的,俯冲板片熔体的参与形成了高氧逸度富金属的岩浆,其对区内成岩和成矿过程具有重要意义。     

西藏冈底斯南部陆陆碰撞早期成矿作用分析

冈底斯带南部发育有大量的斑岩铜钼矿床和矽卡岩型铜铅锌多金属矿床,形成了斑岩铜矿带和多金属矿带.前人的研究表明,成矿带内的矿床形成年代大都小于30Ma,处于碰撞后期伸展构造环境.本文对冈底斯带中南部的甲龙矽卡岩型铁矿、撒当金银矿床(点)和多底沟矽卡岩型钼矿床(点)开展了年代学研究,结果显示:甲龙铁矿黑云母二长花岗斑岩的锆石LA-ICP-MS U-Pb年龄为61.1 ±0.4Ma,MSWD=0.94;撒当赋矿安山岩锆石LA-ICP-MS U-Pb年龄为62.6±0.5Ma,MSWD=1.51;多底沟钼矿床(点)3件辉钼矿Re-Os模式年龄为64.3±0.8Ma ～ 69.2±3.3Ma,加权平均模式年龄为66.7±6.4Ma(MSWD=8.1).三个矿床(点)的同位素年龄表明成岩成矿事件和印度-欧亚板块陆陆碰撞早期构造岩浆事件有关.结合前人工作,我们提出冈底斯中南部发生了大规模与陆陆碰撞早期岩浆事件有关的成矿作用,形成了大面积分布的矿床,具有良好的找矿前景,应引起更多关注.     

Origin of the Dexing Cu-bearing porphyries,SE China: elemental and Sr–Nd–Pb–Hf isotopic constraints

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7044?0.7047, ?_Nd(T) values of –1.5 to?+0.6, and ?_Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.41 and ²⁰⁷Pb/²⁰⁴Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.     

广东大宝山多金属矿床花岗岩锆石LA-ICP-MSU-Pb定年及其地质意义

广东大宝山矿床位于南岭花岗岩带中带。它是我国著名的大型多金属矿床,开采历史久远。近年来的研究表明大宝山矿床与成矿作用有关的斑岩体为燕山早期岩浆活动的产物,因而人们较多地关注中生代的岩浆活动,而忽视了对其他时代岩浆活动的研究。本文在前人研究的基础上,利用锆石LA-ICP-MS U-Pb定年方法系统地测试了大宝山多金属矿床多个花岗质岩体和辉绿岩脉的形成时代,研究表明徐屋片理化流纹斑岩年龄为426.9±2.2Ma、九曲岭黑云母花岗闪长斑岩、船肚花岗闪长岩和大宝山花岗闪长斑岩形成时代分别为162.2±0.7Ma、160.2±0.9Ma和161.0±0.9Ma。矿区内两条辉绿岩脉的年龄分别为210.4±1.4Ma和163.9±1.8Ma。这些结果证实大宝山矿区内存在加里东期、印支期和燕山期等多个旋回的岩浆活动,中晚侏罗世铁镁质的岩浆活动可能存在对成矿的贡献。     

Palaeozoic porphyry Cu–Au and ultramafic Cu–Ni deposits in the eastern Tianshan orogenic belt: temporal constraints from U–Pb geochronology

During late Palaeozoic time, extensive magmatism and associated ore deposits were developed in the eastern Tianshan orogenic belt (ETOB), Northwest China, which is part of the Central Asian Orogenic Belt. To understand the petrogenesis of the intrusions in this area, we performed in situ zircon U–Pb and Hf isotopic analyses on the Tuwu–Yandong (TW–YD) stocks and the Xianshan, Hulu, Luodong, and Poshi batholiths. Two major suites of intrusive rocks have been recognized in the ETOB: (1) 338–339 Ma plagiogranite porphyries and 265–300 Ma ultramafic and mafic rocks, of which the former are associated with 323 Ma porphyry Cu–Mo deposits and have enriched radiogenic Hf isotopic compositions (?_Hf(t) = +11.5 to +15.6), which were derived from a depleted mantle source, whereas the latter are associated with 265–300 Ma magmatic Ni–Cu deposits and have variable Hf isotopic compositions (?_Hf(t) = ?10.3 to +14.3), indicating an origin via the hybridization of depleted mantle magma and variable amounts of ancient lower-crustal components. The proposed magma sources, combined with the geochemical differences between these two suites of intrusive rocks, indicate that in the lower to middle Carboniferous, a N-dipping subduction zone beneath the Dananhu arc triggered the emplacement of granitic porphyries in the Tousuquan and Dananhu island arc belt in the east Tianshan, leading to the formation of the TW and YD porphyry Cu–Mo deposits. In the Upper Carboniferous to Lower Permian, large mafic–ultramafic complexes were emplaced during the closure of the ancient Tianshan Ocean, resulting in the formation of several magmatic Cu–Ni sulphide deposits.